Final answer:
Train operators should assume extreme physical danger or a possible hazard if a proceed buzzer sounds but the indication light is dark, and take necessary safety precautions before advancing.
Step-by-step explanation:
The safest assumption for a train operator to make when he receives a proceed buzzer but his indication light remains dark is that there is a potential for extreme physical danger or a serious fire or hazard ahead. The procedure in such a case should involve verifying the signal with a dispatcher or taking precautionary measures before proceeding. The operator must be aware of the uncertainty and be prepared for any discrepancies between the buzzer and the light, which could indicate a malfunction or a dangerous situation on the tracks.